Currently, there are systems that enable aircraft to follow a precise trajectory on an airport. They are particularly critical when:                the size of the landing gears is close to the size of the lanes on which the aeroplane is taxiing;        the trajectories are curved;        visibility is reduced.        
Notably, in the case of the Airbus A380, the size of the gears is approximately 17 metres and the size of a traffic lane is 23 metres and the first point that the pilot can see is 20 metres from the nose.
To address the aircraft guidance constraints during the ground taxiing phases in an airport, one solution that exists on the A380 is to equip the cockpit with a video system enabling the position of the gears to be seen under the fuselage.
However, other solutions make it possible to generate a particular symbology projected onto a “head up” display screen. Some of these systems are described in the following patents:                “OptoElectronic Device for assisting aircraft taxiing comprising dedicated imaging”, the reference of which is CA 2613740;        “Guiding and Taxiing assistance optoelectronic device for an aircraft having a dedicated symbologie”, the reference of which is WO 2008043763;        “Airport surface operation advisory system”, the reference of which is U.S. Pat. No. 6,571,166 B1;        “Aircraft surface operations guidance on head up display”, the reference of which is U.S. Pat. No. 7,382,284 B1.        
In the first patent, the reference of which is CA 2613740, a so-called “head up” display screen shows a projected 2D plan view which makes it possible in particular to position the gears relative to the traffic lanes, which are also referred to, in aeronautical terminology, as taxiways.
The pilot can then, while taxiing, assess the situation of the aircraft relative to the runway or to the taxiways.
One drawback is that this solution, even though it provides a good control tool, does not make it possible to manoeuvre the aircraft easily in turns.
The second patent, the reference of which is WO 2008043763, is based on the guidance point concept.
Such a guidance point is specific to each type of aircraft and is situated between the nose wheel and the main landing gear. In practice, for large aircraft, neither the nose wheel nor the main landing gear can follow the centre line in the curves without one of the gears being located outside the taxiway. However, the calculation of a guidance point makes it possible to guide the aircraft on the taxiway by keeping the ground projection of this point on the centre line. When the projection of the guidance point is substantially on the centre line, then the landing gears and the nose wheel are assured of not departing from the taxiway.
In FIG. 1, the guidance point G is projected onto the centre line at a point P. The system makes it possible to calculate the angular deviation 4 and represent this deviation in a guidance symbology on a head up display screen.
The point A situated in front of the aircraft, said aircraft following a heading 2, represents the position of the nose wheel. The landing gear 1 comprises a centre A′ situated on the longitudinal axis of the aircraft. The point T is an early point making it possible to pilot, notably in a turn 3, the position of the point G with a margin for manoeuvre. The point T′ is the projection of the point T on the centre line. This system makes it possible to keep the landing gear and the nose wheel of the aircraft on the taxiway.
This mechanism is described more specifically in the patent WO 2008/043763 A1.
When the pilot uses the position of the point G to guide the aircraft in the taxiing phases, one drawback of this system is that the point G does not make it possible to guide the aircraft with an anticipation time, the pilot is always making up a deviation between the centre line and the position of the point G.
When the pilot uses the position of the point G to guide the aircraft in the taxiing phases, one drawback of this system is that the position of the point T makes it possible only to alert the pilot when a trajectory deviation becomes too great. This point does not make it possible to guide the aircraft from a setpoint. As previously, the pilot is always making up a trajectory deviation. Consequently, piloting is uncomfortable and is handled in fits and starts when the aircraft drifts from its trajectory.
In the third patent, the reference of which is U.S. Pat. No. 6,571,166 B1, a system uses another guidance symbology representing a taxiing director representing the current centre line of the taxiway, and an aeroplane symbology situated either side of the centre line relative to the position of the centre of the fuselage of the aircraft likened to the guidance point.
However, this symbology does not make it possible to anticipate the manoeuvres in a turn. In practice, the pilot controls the nose wheel and becomes aware only after a delay of the effects of his manoeuvres on the guidance symbol.
Furthermore, for the large carriers of A380 type or even the Airbus A340-600 family of aircraft, it is sometimes necessary to have both the nose wheel and the rear gears off-centre in a tight turn in order to keep all the wheels on the taxiway. This situation requires the pilot to have an early awareness of the movement of the wheel and of the gears. This constitutes a drawback and a risk for the safety of the aircraft.
The fourth patent, the reference of which is U.S. Pat. No. 7,382,284 B1, introduces a guidance symbology in the form of a conforming view and in the form of a non-conforming 2D view.
The symbology is based on the anticipation of a point called “control point”, and it uses two items in the display of the representation of the taxiway and of the aircraft.
First of all a target, denoted “taxi guidance cue”, indicates the required position of the control point if the trajectory is to be followed. And secondly, a vector, denoted “trend vector”, indicates the estimated position of the control point.
The aim, for the pilot, is to place the point of the “trend vector” in the “taxi guidance cue”.
However, guidance on the ground has precise characteristics that pose problems for this type of symbology.
In practice, on the ground, the pilot has external references such as the centre lines, also considered and denoted in aeronautical terminology as “guidance lines”, painted in the centre of the taxiways. Said guidance lines constitute a natural reference for the pilot enabling the aircraft to be piloted on the taxiway.
However, there may be a conflict between the trajectory of a centre line and the guidance symbology. In particular, this case arises when the pilot has to consciously manoeuvre the nose wheel away from the centre line in a turn in order to keep his landing gear on the taxiway. Notably, the pilot could feel disturbed by the fact that the guidance symbology is no longer totally in agreement with the plot of a centre line.
Furthermore, the ground taxiing constraints are severe; notably, certain turns imposed by the topology of the airport are much tighter on the ground, with a radius of curvature of around 50 metres. Thus, one drawback of this solution is that on approaching such turns, the “taxi guidance cue” is displaced at a stroke, very rapidly, and may even depart from the pilot's field of vision. Keeping the “trend vector” in the “taxi guidance cue” then becomes a very arduous task for the pilot. The pilot has the impression of “running” after his guidance symbol without being able to catch up with it.
Finally, the symbology proposed in this last patent requires a route to be followed to be entered into the system, said route to be followed also being called “taxiing plan”, in order to be able to generate the taxi guidance cue along the trajectory to be followed.
On the ground, the route to be followed on the airport is dictated by the controller to the crew, this route also being referred to in aeronautical terminology as “cleared route”. One drawback lies in the fact that, in most flight management systems, the interfaces allow a flight plan to be entered but, at the present time, there is no system enabling a cleared route to be entered into the aeroplane system.
The invention makes it possible to mitigate the abovementioned drawbacks.